linux-kernel - Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect task->comm access

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Date:	Tue, 17 May 2011 15:56:24 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Ingo Molnar <mingo@...e.hu>, John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Joe Perches <joe@...ches.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Andy Whitcroft <apw@...onical.com>,
	Jiri Slaby <jirislaby@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect
 task->comm access

On Tue, 17 May 2011, Peter Zijlstra wrote:

> The changelog also fails to mention _WHY_ this is no longer true. Nor
> does it treat why making it true again isn't an option.
> 

It's been true since:

	4614a696bd1c3a9af3a08f0e5874830a85b889d4
	Author: john stultz <johnstul@...ibm.com>
	Date:   Mon Dec 14 18:00:05 2009 -0800

	    procfs: allow threads to rename siblings via /proc/pid/tasks/tid/comm

Although at the time it appears that nobody was concerned about races so 
proper syncronization was never implemented.  We always had the 
prctl(PR_SET_NAME) so the majority of comm reads, those to current, 
required no locking, but this commit changed that.  The remainder of comm 
dereferences always required task_lock() and the helper get_task_comm() to 
read the string into a (usually stack-allocated) buffer.

> Who is changing another task's comm? That's just silly.
> 

I agree, and I suggested taking write privileges away from /proc/pid/comm, 
but others find that it is useful to be able to differentiate between 
threads in the same thread group without using the prctl() for debugging?
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